THIS invention relates to electronic identification systems including an interrogator and a plurality of transponders. The invention more particularly relates to transponders forming part of such a system.
Known electronic systems of the aforementioned kind include an interrogator including a transmitter for transmitting an interrogation signal to the transponders and a receiver for receiving a response signal from the transponders. A microprocessor in the interrogator identifies a particular transponder from a data stream in the response signal. Each transponder comprises an antenna and a detector circuit for receiving and collecting power from the interrogation signal, to present a high enough voltage on a storage capacitor, to power a modulator and logic circuitry of the transponder, which logic circuitry in turn generates the aforementioned data stream. The data stream is used to modulate the energizing signal at about 100% modulation depth and to reflect back to the interrogator a portion of the energy in the energizing signal, by what is known as backscatter modulation. The antenna of the transponder is normally a single element half wavelength dipole antenna having a feedpoint impedance of 50.OMEGA. to 100.OMEGA.. This antenna is matched by a suitable impedance matching network to a low input impedance of 125.OMEGA. to 200.OMEGA. of the detector circuit.
In the known systems, the effective distance of the backscattered response signal greatly exceeds the distance over which the transponders can be powered by the energizing signal. The inhibiting factor is the voltage required on the capacitor to power the modulator and logic circuitry of the transponder. The voltage recovered and thus the operating range achievable with the known low input impedance transponders, especially those with a small integrated storage capacitor and which modulate the energizing signal at about 100% modulation depth, are not satisfactory.